Suspension systems for vehicles improve the handling and control of the vehicle by absorbing energy associated with uneven terrain due to bumps, depressions, obstacles, and other such features. Various forms of hydraulic suspension systems have been designed to meet the handling and control requirements of the rider. These systems typically consist of an arrangement of two telescoping tubes, two chambers for holding a viscous fluid, seals for keeping the viscous fluid within the chambers, a damper assembly which separates the two chambers, and a damper valve which regulates the flow of the fluid from one chamber to the other.
In a typical arrangement, an outer tube is fastened to the damper assembly at a point on the upper portion of the bicycle and fits over a lower inner tube which is fastened to a point on the lower portion of the bicycle. The tubes are arranged to allow them to slide axially in a telescoping fashion in relation to each other. The tubes encompass two chambers which hold a viscous fluid. A seal surrounds the upper portion of the lower tube to keep the fluid within the chambers.
When the vehicle passes over a bump, the outer tube slides axially in a telescoping fashion toward the inner tube. The viscous fluid flows from the lower chamber through the damper valve to the upper chamber to allow the outer tube and damper assembly to slide toward the inner tube. During the subsequent expansion phase, the outer tube slides axially in a telescoping fashion away from the inner tube. In the expansion phase, the viscous fluid flows in the opposite direction through the damper valve to allow the outer tube to move away from the inner tube.
Hydraulic suspension systems exhibit a typical dampening performance. If a small input compressive force is slowly and continuously applied to the system, the viscous fluid will flow through the damper opening and the outer and inner tubes will move axially toward each other. Conversely, if a large input compressive force is applied suddenly to the system, the viscous fluid will not be able to flow through the opening fast enough to allow a rapid relative movement of the two tubes. Accordingly, hydraulic suspension systems exhibit more resistance to large, sudden forces than to small, slow forces.
While hydraulic suspension systems typically exhibit the dampening performance described above, the actual dampening performance of a particular suspension system is a function of the physical characteristics of that system. The amount of resistance exhibited by the hydraulic suspension system depends on the rate at which the viscous fluid can flow through the damper valve from the lower chamber to the upper chamber. A suspension system will exhibit less resistance or stiffness in response to a bump if the viscous fluid is permitted to flow more easily through the damper valve. Thus, a hydraulic suspension system with a larger opening between the two chambers will offer less resistance than another system which has a smaller opening.
The prior art has examples of hydraulic suspension systems with telescoping tubes with added features which enable the system to modify the damping performance of the device to a limited degree. As explained in U.S. Pat. No. 4,971,344 to Turner, the suspension system can be designed to exhibit greater resistance to low input forces which could be produced from the pedal force of the rider and lower resistance to high input forces associated with a large bump. In this arrangement, the damper opening is blocked with a plate-like shim until the fluid pressure in the lower chamber becomes greater than the resistance provided by a spring which holds the plate over the opening. This scheme allows the suspension system to absorb energy associated with bumps while preventing the system from absorbing energy associated with pedaling.
As indicated by this discussion, the hydraulic suspension systems in the prior art have two major limitations. First, they have limited, static control over the dampening performance. For example, prior art hydraulic suspension systems exhibit smaller resistance throughout their operating region by using a larger opening between the two chambers. Similarly, the Turner suspension system exhibits stepped resistance, one level of resistance when the input force is lower than a certain threshold value and a second, higher level of resistance when the input force exceeds the threshold value. Second, the prior art offers the rider only a limited ability to change the dampening performance of the suspension system to match the rider's preference. For example, the rider of the Turner suspension system can adjust the gas pressure in the upper chamber to vary the resistance.